Milk fat (also commonly known as butter fat or butter oil) comprises the oily portions of the milk of mammals. Cow's milk, which is a common source of milk fat, contains approximately four weight percent milk fat. Milk fat is comprised primarily of triglycerides, e.g., about 98 weight percent, although some diglycerides and monoglycerides may also be present. The fatty acids from the glycerides in milk fat typically contain from 4 to 18 carbon atoms. In addition, typically about 30 to 40 weight percent of the fatty acids are unsaturated, i.e., contain one or more double bonds.
Low-fat and skim milk-based products are made by removing milk fat from whole milk. As a result of the high commercial demand for low-fat and skim milk-based products, there is currently a large supply of milk fat which is available for other uses.
Various components of milk have been proposed for use in personal care compositions. For example, the protein fractions of milk are known to have the ability to hydrate skin and help retain water. Whey proteins are known to have good gelling properties. Casein is often used in personal care compositions for its emulsification and foaming properties. Even whole milk is used to enhance the cleansing effect of surfactants in personal care compositions.
The use of milk fat also has been proposed in personal care compositions. However, the use of milk fat in personal care compositions has been limited due to problems with rancidity which can cause off-color, offensive odor and poor taste in the personal care compositions such as, for example, in lipsticks.
The rancidification of milk fat is dependent on the action of two kinds of chemical transformation: lipolysis and autoxidation. Lipolysis is the process of enzymatic hydrolysis of glyceride esters and is associated in dairy products with the appearance of rancid flavor due to the accumulation of free fatty acids. Lipases are the enzymes capable of catalyzing this hydrolysis and are often dormant until stimulated by a change in the milk, such as dilution, mechanical, thermal or chemical reaction. The enzyme activity can be decreased by the use of small amounts of salt and by pasteurization treatment. Ultraviolet and gamma ionizing radiations also deactivate lipase by chemical modification of the enzyme. Autoxidation occurs by a free-radical chain mechanism which includes initiation, propagation and termination, resulting in the formation of hydroperoxides which can collapse to stable carbonyl and hydroxy compounds, or react with other components in the personal care composition.
In view of the current commercial availability of milk fat, new modified milk-fat products are desired which can provide increased resistance to rancidification. Such milk-fat products could be used, for example, in a variety of personal care compositions.